Spontaneous glass breakage is a phenomenon by which toughened glass may
spontaneously break without any apparent reason. The most common causes are:
Minor damage during installation such as nicked or chipped edges later developing
into larger breaks normally radiating from point of defect.
Binding of the glass in the frame, causing stresses to develop as the glass
expands and contracts due to thermal changes or deflects due to wind
Internal defects within the glass such as nickel sulfide inclusions
Thermal stresses in the glass Inadequate glass thickness to resist
wind load Installation damage
While glass is being moved and installed, it is easy for the glaziers
to nick or chip the edges of the glass with various tools. It is also possible
for fasteners such as nails or screws used to attach glass stops to nick the
glass edges if these fasteners are installed at an improper angle. These
small nicks or chips may not result in immediate breakage. However, over time,
as the glass expands and contracts, stress concentrations can develop around
the nick, leading to breakage. In the case of tempered glass the entire unit
usually breaks. Binding in the frame
Glass expands and contracts with changes in temperature and deflects due to wind,
so almost all modern glass is set on resilient blocks at the bottom and with
space for expansion at the sides and top. The gaskets holding the glass in
the frame are also usually resilient to cushion the glass against wind
buffeting. If no space is provided at the perimeter of the unit, the glass
will bind against the frame, causing internal stresses to develop in the
glass which can exceed the strength of glass, resulting in breakage.
Internal defects & inclusions in the glass
Nickel sulfide inclusions can be present in the glass. The most common cause of
these inclusions is the use of stainless-steel machinery in the
glassmaking and handling process. Small shavings of stainless steel containing
nickel change structure over time and grow, creating internal stresses in the
glass. When these stresses exceed the strength of the glass, breakage results.
This type of breakage is almost always found in tempered glass and is indicated
by a distinctive "figure eight" pattern, with each "loop" of the figure eight
approx. 30mm in diameter. Alternatively, small pieces of
refractory brick can be eroded by the molten glass from the internal walls of
the furnace during processing and become embedded in the finished glass. These
are also known as "stones", and can also break the glass when the glass is
heated, as they create thermal anomalies.
Thermal stresses Breakage due to thermal stress is most
common in large pieces of sealed insulating glass with heavy
heat-absorbing coatings. The coating is usually applied to the "number two"
surface. This causes the outside lite of glass to heat up more than the inside
lite as the coating converts radiant heat from the Sun into sensible heat. As
the outer lite expands due to heating, the entire unit bends outward. If the
spacer bar or other edge condition connects the two lites of glass in a
very rigid manner, bending stresses can develop which exceed the strength of the
glass, causing breakage. This was the cause of extensive glass breakage at the
John Hancock Tower in Boston. Inadequate glass thickness
A pane that is too large or thin, having not been properly engineered for wind
loads on the site, can be broken by the wind.
Remedies Any breakage problem has more severe
consequences where the glass is installed overhead or in public areas. A
safety window film can be applied to the tempered panes of glass to protect from
its falling. An old-fashioned precaution was to install metal screens below
skylights. References
American Society for Testing and Materials
ASTM E 2431 -- Practice for Determining the Resistance of Single Glazed Annealed
Architectural Flat Glass to Thermal Loadings.
ASTM E1300 -- Standard Design Practice for Determining Load Resistance of Glass
in Buildings.